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Special Climate Central Report on Best Cars for Climate

Published: April 25th, 2012

Methodology

Our calculations begin with the EPA’s combined highway/city driving fuel economy of cars: miles per gallon for gasoline cars and kilowatt-hours per mile for electric cars. (See Appendix Table A1 of Full Report)

For a gasoline car, the bulk of the lifecycle greenhouse gas (GHG) emissions associated with driving are due to the CO2 emitted by combustion of the fuel in the car’s engine. A gallon of gasoline releases about 19 lbs of CO2 when burned, or about three times the weight of the gallon before it burns. To these CO2 emissions we must add the GHG emissions associated with extracting, transporting, and refining the crude oil used to make that gallon of gasoline. When these are included, the total lifecycle GHG emissions for using gasoline in a car come to 25.9 lbs of CO2-equivalent per gallon. (a)

In this report, the term CO2-equivalent (or CO2e) is used to refer to GHG emissions. This measure expresses the combined global warming impact of several different gases in terms of the amount of CO2 alone that would give the same warming. (GHGs in addition to CO2, such as methane (CH4) and nitrous oxide (N2O) are emitted over the lifecycles considered here.) Since different gases have different lifetimes in the atmosphere, the relative warming impact of the non-CO2 molecules depends on the time frame under consideration. The Fourth Assessment Report of the Intergovernmental Panel on Climate Change (b) gives global warming potentials (GWPs) relative to CO2 for a large number of gases considering 20-year, 100-year, and 500-year time frames. For the results reported in the main body of this report, we have used the 20-year GWP values since 20 years is close to the typical lifetime of a car — certainly much closer than either 100 years or 500 years. Our results recalculated using 100-year GWP values are given for comparison in the Appendix (Tables A2 and Table A3 of Full Report). (For a gasoline car, the lifecycle emissions assuming a 100-year GWP are 24.3 lbs CO2e/gallon instead of 25.9 for a 20-year GWP. (a))

Estimating GHG emissions associated with electricity use by an electric car is more difficult than estimating emissions for a gasoline car, because it is essentially impossible to say with certainty that an electron generated at a particular power plant is the same electron that ends up in the battery of a particular vehicle. The uncertainties arise because of the nature of electricity flow and the geographical extent and interconnectedness of electricity grids. (c) Additional uncertainty is introduced by the time-varying nature of electricity demand and supply. For example, if an electric vehicle plugs in to charge during a period of peak electricity demand, the mix of power plants generating electricity (and hence the GHG emission profile of the electricity) will be different from the mix of plants during periods of lower electricity use. In general, the greater the temporal or geographic specificity with which we wish to determine the emissions associated with electricity use, the greater will be the uncertainty around whether the emissions accurately represent actual use.

To make our analysis tractable, we have chosen not to consider time-of-use variations in electricity emissions, choosing instead to use annual emissions per megawatt-hour generated from power plants. We also assume electricity generated in a state is consumed in that state. A recent similar study by the Union of Concerned Scientists (d) also uses annual emissions per megawatt hour, but chooses to divide the U.S. into 26 electricity-generating/consuming sub-regions defined by the EPA. (e) Another study in 2007 by the Electric Power Research Institute and the Natural Resources Defense Council divided the U.S. into 13 sub-regions. (f)

The larger the geographic region selected, the more certain one can be of the average emissions associated with each kilowatt-hour used in that region — for example, the average emissions per kilowatt-hour consumed for the entire U.S. can be known with considerable certainty. The drawback of averaging over larger and larger areas is that less and less insight can be gained into the impact of geographic distribution of different electricity generating sources. In an effort to balance these competing considerations, we have chosen to average emissions at the state level. For large states, or for states of any size that have similar electricity generating fuel mixes as neighboring states, the uncertainty introduced by this assumption is small. The uncertainties are larger for smaller states.

To estimate state-level GHG emissions associated with electricity, the following methodology was adopted. The starting point were data published by the Energy Information Administration (EIA) on how much CO2 was emitted on average per kilowatt-2 hour (kWh) of electricity generated in each state in 2010 (TableA4 and Figure A1 of Full Report). (g) This average is most influenced by the types of fuels used in the power plants in the state. For example, a state that relies more on nuclear or hydro power will have lower average CO2 emissions per kWh generated than a natural gas-reliant state or, especially, a coal-reliant state. But CO2 emissions at a power plant alone are not the full emissions story because there are also emissions associated with supplying fuel to the plant (e.g., emissions that occur during coal mining or natural gas extraction). Accurately estimating on a state-by-state basis the emissions other than those at the power plant itself requires detailed lifecycle calculations.

These calculations were undertaken using the Argonne National Laboratory’s Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation (GREET) Model, version 1_2011, the same model used to estimate the lifecycle GHG emissions for gasoline mentioned above. GREET was run for each state’s electricity system by specifying in GREET the mix of fuels used for electricity generation in the state. (g) The main sources of electricity in any given state in the U.S. are coal, natural gas, nuclear, and/or hydro. (Renewables other than hydro play small roles in most states today.) In the case of natural gas, the power plant technologies used vary significantly from state to state, and the average efficiency of generation from natural gas varies accordingly from state to state. (This is not the case for coal, nuclear, or hydro plants.) Efficiency directly impacts the GHG emissions per unit of electricity produced, so we provided the mix of natural gas power plant technologies in each state as an input to GREET. The mix of natural gas powerplant technologies (combined cycle, simple cycle, or steam cycle) in each state was obtained from EIA data. (h) GREET’s default values for electricity generating efficiencies were then kept for all power plant technologies.

The outputs from running the GREET model for each state include A, the average CO2 emissions at power plants per kWh generated and B, the average total lifecycle GHG emissions in CO2- equivalents per kWh delivered to the end user. (The transmission and distribution losses assumed by the GREET model are 8% of generated electricity.) For each state, the ratio B/A was calculated and multiplied by the average CO2 emissions per kWh of electricity generated (derived from EIA data, as described above) to arrive at the lifecycle GHG emissions associated with electricity used in each state in 2010. For each state, separate calculations were done using 20-year and 100-yr GWP values for non-CO2 gases (Table A4 of Full Report).

(b) IPCC, 2007: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007. Available at www.ipcc.ch.

(c) For a thoughtful discussion of this issue, see C.L. Weber, P. Jaramillo, J. Marriott, and C. Samaras, “Life Cycle Assessment and Grid Electricity: What Do We Know and What Can We Know,” Environmental Science & Technology, 44: 1895-1901, 2010.

(e) The UCS report uses an overall methodology quite similar to the one we have used. Two notable differences are in some key input assumptions, including the use of 2007 emissions data by the UCS (rather than the more recent 2010 data we have used) and 100-yr GWP values for estimating the global warming impact of non-CO2 greenhouse gases. We have used a 20-yr GWP, but also show results for 100-yr GWP in the Appendix.

(h) Theamountsofelectricitygeneratedineachstatein2010brokendownbytypeofpowerplanttechnologyarecollectedbythe Energy Information Administration on form EIA-923 and published in spreadsheet form. For input to GREET, the natural gas generating technologies were categorized as combined cycle, simple cycle, or steam cycle.